People in TV medical shows tend to have a compromised life expectancy. From the moment a character actor is wheeled into Casualty with an odd cough or strange rash, it’s rarely long before the regular cast members are shouting “Stand clear!” and trying to shock them back to consciousness. Real patients, meanwhile, need to have something pretty nasty to earn screen-time in a hospital reality series.

The shows tend to be sturdy veterans of the schedules. BBC One has three medi-soap Methuselahs: Casualty is 32 years old, Holby City 19 and Doctors 18. The runs of that trio alone almost match the NHS’s 70 years. Documentaries about docs are equally long-lived: 24 Hours in A & E, is currently in its 15th run, while a previous fly-on-the-ward documentary, Jimmy’s, filmed at St James’s Hospital in Leeds, spanned 12 series.

The reason TV enjoys playing doctors and nurses is that the subject delivers the universal relevance of which commissioners dream: it’s rare a UK viewer could have escaped some exposure to the NHS. In this respect, medical drama is the opposite of that other ratings staple, crime drama. Cop shows dramatise the audience’s fear of becoming a victim of serious crime, even though statistics suggest the risk is individually very low. Medical dramas act out a common horror of hospitalisation for major illness, which, the figures show, will happen to a majority of those watching.

The oldest British broadcasting medical franchise, Dr Finlay’s Casebook, was set in the 1920s before free-health provision. The first NHS drama was ITV’s Emergency – Ward 10, set in the imaginary Oxbridge Hospital. This established the genre as a sort of medical docu-soap, in which, between eruptions in their love lives, characters made clinical interventions that were technically plausible, even if they occurred at a rate that was improbable in a single building. This tradition was followed in the 70s by General Hospital, Angels, Peak Practice in the 90s, and then Casualty, Holby City, and Doctors. (When ITV re-animated Doctor Finlay in 1993, the producers had the smart idea of updating it to 1948, with the central medic returning from service during the second world war and having to adjust to the provision of free healthcare.)

A second, comic line of investigation began with Doctor in the House, based on the autobiographical novels of Richard Gordon, which continued through five spin-offs, taking the medics to places including the sea and Australia, until 1991. The gentle gags in that franchise became harder, with a strain of satirical or polemical shows, starting with Dennis Potter’s Emergency Ward 9, with its titular nod to the ITV hit.

Jed Mercurio brought his own experience as a hospital doctor to Cardiac Arrest, a daring, dark comedy in which the central character, Helen Baxendale’s Dr Claire Maitland, was a demonic exception to the usual angelic template of TV medics. Mercurio’s Bodiesand Critical also depicted a national health service in which institutions and staff were on the brink of breakdown. He implicitly used medicine as a metaphor for other failings of the state, as had GF Newman in his pointedly titled four-parter, The Nation’s Health, which, in the year of Margaret Thatcher’s second election victory, had episodes boldly titled: Acute, Decline, Chronic, and Collapse.

This mixing of healthcare drama with politics has frequently made the genre controversial. When the first TV version of Dr Finlay’s Casebook was shelved in 1971, a delegate to a medical conference said the lead character “should be in horror movies” because of his terrible track record of treating patients and that the show being dropped was “the best thing to happen for the good name of public health”. Doctors have criticised hospital programmes for exaggerating the possibility of emergency resuscitations: a rare intervention medically, but routine televisually. However, medical professionals have regularly voted Cardiac Arrest the fiction that most captures the fact of being on the NHS frontline.

Factual series about the NHS have proved almost as durable as dramas. Apart from Jimmy’s and 24 Hours in A&E, numerous other similar prescriptions include Junior Doctors: Your Life in Their Hands, Hospital 24/7, Ambulance and Hospital.

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The sub-title of Junior Doctors claims continuity with a long-running BBC medical factual franchise, Your Life in Their Hands, which began in 1958. That name spelled out the premise – this will probably be you one day – that underlies all docs about docs. The strength of these shows is that the stakes of real illness and death are higher than the most authentic drama can achieve, but the participation of real people is also the genre’s weakness. Although fictional and factual medical shows largely celebrate the heroic dedication of NHS staff, both can make uncomfortable viewing for health secretaries and NHS managers. Hospital, filmed at Nottingham University hospitals, made a significant political and journalistic impact by showing the consequences – a daily lottery for beds and operating slots, surgery cancelled because of overcrowding – of funding decisions and a tough winter.

Casualty and Holby City have often been rebuked by politicians for loaded storylines: the first two series of Casualty centred on the fight to save night-shift staffing from Thatcherite cuts. In 1993, the show was censured by the regulator, the Broadcasting Standards Council, for a plotline involving rape, and by a BBC internal review for storyline in which rioters burn down a ward. In 2012, Holby City controversially engulfed the whole hospital with rioting resulting from a protest over police brutality.

Paul Unwin, co-creator of Casualty, has said that the series was specifically intended to be pro-NHS, anti-Conservative, feminist and anti-racist. And, regardless of how that ambition may have stretched BBC editorial guidelines, it was true to the genre’s history.

One of the most commendable aspects of British medi-drama is that it has always been far ahead of the rest of the medium in gender and racial representation. The British Caribbean actor Joan Hooley and Pik Sen-Lim, a Malaysian Chinese performer, had decent roles in Emergency – Ward 10 at a time when TV drama casts tended to be all white. Kwame Kwei-Armah, now a playwright and artistic director of the Young Vic Theatre in London, had a leading featured role in more than 150 episodes of Casualty in an era when it was shamefully rare for a black actor to achieve such visibility on British television.

In many cases, such progressiveness was an accidental consequence of the ambition for authenticity, but Angels, a bi-weekly soap opera about NHS nurses, used realism to sneak radical casting past the BBC. Creator Paula Milne realised that focusing on a profession dominated by women gave the opportunity for a long-running drama with six female leads; it remains one of the few peak-time dramas to have had to introduce token male characters to even things up. The District Nurse and Call the Midwife were also able to feminise cast lists by reflecting professional reality.

Universal and timeless in their focus on the fight to stay alive – but also effortlessly contemporary in the range of people they feature – medical series of one sort or another seem likely to remain a key way for schedulers to stop their ratings flatlining.